There are a number of devices which use materials having non-linear electrical response (hereinafter non-linear material) for overvoltage protection. These devices use non-linear material comprising finely divided particles dispersed in an organic resin or insulating medium. The material is placed between contacts and responds or switches at predetermined voltages. U.S. Pat. No. 4,977,357 is directed to such a material which can be placed between and in contact with spaced conductors to provide a non-linear resistance therebetween; the material comprises a matrix comprised of a binder and closely spaced conductive particles uniformly dispersed in the binder. U.S. Pat. No. 4,726,991 is directed to a switching material which provides electrical overstress protection against electrical transients, the material being formed of a matrix comprising separate particles of conductive materials and semi-conductive materials, all bound in an inorganic insulating binder to form the switching matrix. U.S. Pat. No. 3,685,026 describes a switching device employing a non-linear material.
In all such devices, the matrix has been applied between electrodes by forming the matrix material into the space between the electrodes, by applying a coating of the material to one electrode and then applying the second electrode, or by extruding, rolling/calendaring, pressing or molding the material into a thin sheet which is then sandwiched between electrodes. In all such methods, it is difficult to precisely achieve the desired thickness of the non-linear material and to provide intimate contact with the associated electrodes.
In copending application U.S. Ser. No. 07/949,709 filed Sep. 23, 1992, now U.S. Pat. No. 5,262,754 there is described an overvoltage protection element including a perforated layer of insulating material with the perforation filled with nonlinear material. The thickness of the nonlinear material is controlled by the thickness of the layer and the switching characteristics by the material selected. The perforations are formed by processing the layer of material. There is a need for an insulating layer which does not require processing, thereby lowering the cost of the element and simplifying the manufacture.